In cognitive radio, which corresponds to a radio communication system in which parameters to be used for radio communication are adaptively changed according to a surrounding radio environment, a surrounding radio environment is cognized (radio signals are detected) and parameters are optimized according to the radio environment. In particular, as a secondary system, a cognitive radio communication system shares and uses a frequency band that is allocated to another radio communication system (hereinafter, referred to as primary system), to thereby improve efficiency of the frequency band usage.
In sharing a frequency band between the primary system and the secondary system, it is necessary to protect the primary system preferentially. Accordingly, the primary system may preferentially use a frequency band that is allocated thereto in advance. Further, in order to avoid interference with the primary system, the secondary system is required to use a frequency band that is not used by the primary system, or to perform communication so that its interference amount is equal to or smaller than an interference amount allowed by the primary system. In other words, the secondary system needs to correctly recognize, before the use of the frequency band, a status of the band usage of the primary system.
Methods of recognizing the status of the frequency band usage by the secondary system may be classified into two main types.
One type corresponds to a method in which a radio apparatus of the secondary system detects, before starting communication, a frequency band that is being used by the primary system in a candidate frequency band to be used by the secondary system. The other type corresponds to a method in which the secondary system detects that the primary system has started communication in a frequency band that is being used by the secondary system. In this case, the secondary system stops using the frequency band and performs communication using another available frequency band.
As a specific method of detecting a frequency band used by the primary system (frequency band that the secondary system intends to use), there is spectrum sensing as means for the secondary system radio apparatus (cognitive radio apparatus) to detect a radio signal in its surrounding. Spectrum sensing mainly includes a method based on power detection in which determination is made based on a magnitude of a received signal power determined based on a time average, and a method in which a feature amount contained in a transmitted signal of the primary system is used for detection. As the method using a feature amount of the signal, there are provided a method using cyclostationarity contained in the transmitted signal of the primary system, and a method in which the secondary system radio apparatus prepares the same signal sequence as a pilot signal sequence contained in the received signal to correlate the signal sequence to the received signal sequence. Those technologies are described in, for example, Non-patent Document 1 and Patent Document 1.
Patent Document 1 describes a cognitive communication system including radio channel detection means for detecting whether or not a radio channel (radio band) that is allocated to another radio apparatus (primary system) is free. In the cognitive communication system, when a radio channel is detected and turns out to be free (when the primary system is not using the radio channel), an effective period is set, and the expiration of the effective period triggers a radio base station to instruct a radio terminal of the cognitive communication system to detect a frequency band with the expired effective period, to thereby recheck the free state of the radio channel. Further, the document describes a spectrum sensing technology in which a received signal is analyzed to identify a frequency characteristic, an amplitude characteristic, an access scheme, a modulation scheme, and the like.
It should be noted that the spectrum sensing performed by each individual secondary system radio apparatus as described in Patent Document 1 is difficult in reliable detection of the primary system due to influences of a surrounding radio propagation environment, including fading, shadowing, distance attenuation, and frequency correlation. In addition, the spectrum sensing is highly influenced from detection accuracy due to each individual secondary system radio apparatus, failure in detection, and the like.
Next, with reference to FIG. 1, description is given of a concept of cooperative sensing performed by a plurality of radio apparatus of the secondary system. In FIG. 1, there are provided a primary system radio apparatus 100 for transmission and a primary system radio apparatus 110 for reception, which are existing radio apparatus, and secondary system radio apparatus 200 to 230 included in a group (radio apparatus group) including the whole or part of radio apparatus of the secondary system that may use the same communication scheme. FIG. 1 exemplifies a group (radio apparatus group) including four secondary system radio apparatus. One of the secondary system radio apparatus included in the group is classified into a master node 200, and the others are classified into slave nodes 210, 220, and 230. It should be noted that the slave nodes 210, 220, and 230 may have the same configuration as that of the master node, or may have a configuration different therefrom.
Here, the master node 200 and the slave nodes 210, 220, and 230 that are included in the group individually detect a primary system. As a spectrum sensing method to be used for the detection, the method described in Non-patent Document 1 and Patent Document 1 or other such method may be employed with no particular limitation.
The slave nodes each perform detection and then transmit detection information to the master node 200. The master node 200 uses the detection information on the used frequency band, which is received from each of the slave nodes, and detection information obtained through detection performed by the master node 200 itself, to thereby determine whether or not the frequency band on which the detection has been performed is being used by the primary system. The master node 200 notifies the slave nodes of an available frequency band.
As described above, in the cooperative sensing, any method may be used as the spectrum sensing method individually performed by each secondary system radio apparatus. However, as compared with the spectrum sensing based on power detection, the spectrum sensing using a feature amount, such as the method using a correlation with a pilot signal, is higher in accuracy of detection of the primary system, but longer in processing time. Hence, it is not preferred that the spectrum sensing using a feature amount be applied to all channels included in a wide frequency band. Therefore, there is a demand for a technology that enables efficient detection on all channels included in a wide frequency band.
Patent Document 2 describes an example of the technology of performing detection on the above-mentioned wide frequency band. Patent Document 2 describes a method in which two kinds of waiting times different in length are prepared in a channel that is being used by the secondary system, and detection of the primary system is performed at separate two stages. In this method, there are provided, in a predetermined cycle, one or a plurality of short waiting times used in detection at a first stage, and one long waiting time used in detection at a second stage. The short waiting time is associated with each channel, and each secondary system radio apparatus performs, on the channel associated thereto, sensing using spectrum sensing that requires a shorter period of time and has a smaller processing amount, such as the above-mentioned power detection. Based on results obtained therethrough, the secondary system radio apparatus omits the detection at the second stage on channels having sufficiently low possibility of use by the primary system. For example, the detection at the second stage is omitted for channels having power values obtained through the power detection constantly below a predetermined threshold, and other such channels. Next, during the long waiting time, the secondary system radio apparatus performs detection of the primary system with higher accuracy only on the remaining channels by means of the above-mentioned spectrum sensing using a feature amount.
With the above-mentioned method, the spectrum sensing such as the power detection that is shorter in processing time is used at the first stage, to thereby narrow down in advance channels on which the spectrum sensing using a feature amount is to be performed, which results in the shorter processing time.    Patent Document 1: JP 2007-88940 A    Patent Document 2: WO 2007/096819    Non-patent Document 1: D Cabric, S. M. Mishra, and R. W. Brodersen, “Implementation issues in spectrum sensing for cognitive radios,” Proc of the Thirty-Eighth Asilomar Conference on Signals, Systems and Computers (November 2004)